The overt startle reflex and the airpuff elicited eyeblink are subject to modification by events in the sensory environment. Inhibition or facilitation of response amplitude or latency reduction may be seen, depending on the presence or stimulation or the interval between changes in stimulation and the subsequent reflex-eliciting stimulus. One objective of the proposed research is to identify the neural substrate of the startle reflex in rats and pigeons, as well as determining which CNS structures are implicated in the various reflex modification effects. These investigations to be accomplished by pretesting animals, lesioning or ablating appropriate areas, and retesting to identify deficits in the startle reflex, or in its inhibition, facilitation, or latency reduction by visual and auditory prestimulation. Other studies will extend reflex-modification effects previously found with the startle reflex of lower animals to the airpuff elicited eyeblink reflex of humans. This work will examine the effects of various kinds of auditory and visual stimulation on the response to subsequent reflex-eliciting stimulation. Finally, the modification of the eyeblink reflex will be examined in infants and children with the objective of determining the feasibility of reflex inhibition audiometry, an experimental hearing test in which the inhibition of the reflexive eyeblink by prior auditory test stimulus serves as an index of hearing acuity. This procedure will be evaluated with regard to its sensitivity, reliability and diagnostic potential.